Travelling across Ireland, it is hard to ignore the lines of dead and dying ash trees in the hedges.
Ash dieback is a serious disease caused by the fungal pathogen Hymenoscyphus fraxineus. The spores of the fungus blow in the wind and infect ash foliage, turning the leaf-tips brown. It damages the leaves and twigs by making a chemical that is very toxic to the ash.
The fungus produces spores which form sticky masses which then spread through the leaf stalks, down into the branches, trunk and roots, blocking off the tree’s water supply. The leaves wilt and turn brown, lesions appear on the trunk and twigs decay resulting in crown dieback. The spores can only survive in the air for a few days so dispersal is limited.
Even so, the disease has spread rapidly across much of Europe. It was first noted in Ireland in October 2012 on plants imported from Continental Europe and is now recorded in every county. The disease can affect ash trees of any age and in any setting. It can be fatal, particularly among younger trees, while older specimens can hang on a bit longer as they have greater resources at their disposal.
While I will mourn the loss of ash in our own woodland it will also be interesting to see which species takes its place as gaps open up in the canopy. The loss of ash trees is likely to lead to their replacement by sycamore and other species which are not native in Ireland, resulting in changes to the composition of the woods.
Ash is so common in the landscape that the dieback is already having dramatic effects across the country. Almost every hedgerow has some ash and, along with hawthorn, it must be one of the most abundant woody plants in Ireland.
History tells us that ash trees were often grown in important locations of Ireland. The historic kingship of Usnagh had an ash at its centre. Another one, known as Bile Tortan, stood in Co Meath and was regarded in Celtic times as a protection by the gods of the land and its fertility. The appearance of new leaves on the trees is often considered in rural areas as a sign of the summer to come. “Oak before ash, we’re in for a splash; ash before oak we’re in for a soak,” was a traditional country saying that I learned as a child.
I wonder if there is any hope for ash trees in view of the rapid spread of the disease. Is there a resistant strain of the species that will survive the initial onslaught to pass on its genes to the next generation?
A very small proportion of ash trees show natural tolerance to the pathogen. This means that they show minor symptoms and the disease does not have a noticeable impact on their growth or health. Teagasc, the agriculture and food development authority, is working to identify such trees and build up a gene bank with the ultimate goal of producing tolerant ash seed and restore ash trees to Irish forests and hedgerows.
These genotypes will be used to bulk up stocks of tolerant trees and establish seed-producing orchards. They are forming a collection of tolerant genotypes of ash by importing material from different ash breeding programmes in Europe. To date, Teagasc has built a collection of 200 tolerant genotypes. Multiple copies of each genotype have been produced by grafting.
I will be watching our ash trees closely over the next few years to see if there is any inherent resistance here. In a woodland such as ours, where biodiversity is more important than timber production, we will be leaving nature to take its course and it will become apparent quite soon which, if any, of the ash trees are tolerant of the disease.
Whatever happens in the future, it is certain that some plants and animals associated with ash will suffer. Recent research in the UK has produced the most comprehensive account of all plant and animal species known to use ash. A staggering total of nearly a thousand species was listed.
If the dieback is slow, then perhaps the gaps in the canopy of many poorly managed woodlands will help them to diversify by allowing sunlight to reach the forest floor, thus stimulating seedlings to germinate. I am optimistic that nature will find a way around this as it has done with many other plant pathogens.
Richard Nairn is an ecologist and writer. His latest book is Future Wild: Nature Restoration in Ireland
